The present invention relates to a communication system, and a communication sending and receiving device and a communication terminal in the system, and particularly to a communication system suitable for a satellite, a satellite orbit and a satellite orbit configuration algorithm usable in the field of communication and broadcast such as satellite communication, satellite broadcast, satellite mobile communication and in the field of observation with regard to a celestial body of the satellite traveling around, and suitable for a communication and broadcast system to which the satellite is applied, and a satellite communication sending and receiving device and a communication terminal in the system.
(a) The Prior Art with Regard to Setting of an Orbital Element (Argument of Perigee) of an Artificial Satellite
In a case where an artificial satellite is traveling around the earth as the center, the orbit of the artificial satellite always fluctuates under the influence of the nonuniformity of the earth""s gravitational field, the attractive forces of the moon and the sun, the atmospheric drag and the sun""s light pressure. From this viewpoint, the orbit of the artificial satellite traveling around the earth as the center can not be a circular orbit but is an elliptical orbit in a broad sense having a little eccentricity.
Accordingly, the xe2x80x9celliptical orbitxe2x80x9d in the present specification is defined as an xe2x80x9corbit having an eccentricity larger than zero and smaller than 1, and the eccentricity is not intended to become zero in a process of setting an orbital element of the orbit onto which the artificial satellite is injected in order to attain the purposexe2x80x9d.
As an example of practically used artificial satellites having an elliptical orbit, there is the Molniya satellite (about a 12-hour orbital period) which has been used by Russia since the era of the former USSR. As artificial satellites having an elliptical orbit, a communication satellite called as Archimedes (about an 8-hour orbital period) is proposed in Europe. Further, a tundra orbit of about a 24-hour orbital period is proposed though it is not practically used yet. A common point in these satellites is that all the satellites has an orbital inclination angle of approximately 63.4 degrees.
In general, the perigee of an orbit on which an artificial satellite is traveling will rotate on the orbital plane under the influence of the nonuniformity of the earth""s gravitational field (oblateness of the earth""s shape). However, by setting an orbital inclination angle to 63.4 degrees in a mathematical model for calculating a time-varying rate of the argument of perigee, a multiplicative term becomes zero to make the time-varying rate zero. Therefore, it is considered that the rotation is stopped.
(b) The Prior Art with Regard to Orbit Configuration Methods for a Plurality of Artificial Satellites
Communication systems using a plurality of artificial satellites traveling on elliptical orbits have been in practical use and studied. Although arranging of a plurality of artificial satellites in the above-mentioned Molniya satellite and Archimedes is described, no detailed method of arranging the orbits is described.
Although in resent years a communication system using a plurality of artificial satellites is proposed, no detailed technique with regard to orbit configuration method is disclosed. Therefore, a detailed orbit configuration technique is required.
On the other hand, xe2x80x9can artificial satellite on an orbit having a long stretch of time staying in the zenith direction, a method of controlling the orbit and a communication system using the artificial satellite and the methodxe2x80x9d is proposed in Japanese Patent Application Laid-Open No.11-34996.
(c) The Prior Art with Regard to Mobile Communication and Broadcast to a Mobile Object
In the past, when a television broadcast was tried to receive in a mobile object such as a vehicle, there were problems in that the picture came out badly in an area far from a broadcasting facility of a television station, that screen flicker occurred even at a place near the broadcasting facility of the television station, and that receivable channel varied by moving. When a television broadcast from a communication and broadcast satellite on a stationary orbit was received on the mobile object, it was difficult to comfortably enjoy watching television on the mobile object because the electromagnetic wave was frequently shielded by artificial structures such as buildings, trees and natural geographical features.
Transmission of a large volume of data such as images from a mobile object such as an ambulance can not be performed by an existing ground communication infrastructure and an existing communication satellite.
In order to solve the above-mentioned problems, a method of setting orbital elements of an artificial satellite for transmitting a large volume of data to a mobile object such as a vehicle is proposed and in addition an orbital element of the artificial satellite is also proposed in Japanese Patent Application Laid-Open No.11-34996.
The problems in the prior art will be described below, corresponding to each of the above items (a), (b) and (c) based on examples known in the art.
(a) Problems with Regard to Setting of an Orbital Element (Argument of Perigee) of an Artificial Satellite, and an Object of the Present Invention
In the above mentioned Molniya satellite, Archimedes and Tundra orbit, all the orbital inclination angles of them are fixed to approximately 63.4. It seems that the main object is to suppress the rotation of the perigee on the orbit plane. On the other hand, there is an advantage in using an orbital inclination angle as large as approximately 63.4 degrees because the area using the above-mentioned artificial satellites is a higher latitude area such as Europe and Russia.
As for the location of Japan, the territory spreads from a middle latitude to a low latitude as Etorofu island in the northernmost end is situated in latitude approximately 45 degrees north and Okino-torishima in the southernmost end is situated in latitude approximately 20 degrees north. Therefor, when the orbital inclination angle of 63.4 degrees is employed as described above, the artificial satellite system becomes difficult to be used from the territory of Japan unless an altitude of the orbit is sufficiently high. Accordingly, when an orbit of an artificial satellite matching the location of the territory of Japan is taken into consideration, the orbital inclination angle can not help employing a value other than approximately 63.4 degrees, and consequently the perigee of orbit rotates.
In order to control the rotation of the perigee, propellent for controlling the rotation needs to be mount on the artificial satellite. An analytical simulation was performed on a case where the orbital inclination angle is 40 degrees and the eccentricity is 0.24 among the orbital elements proposed in, for example, Japanese Patent Application Laid-Open No.11-34996. As a result, it was found that an amount of the propellent for controlling the argument of perigee becomes dominant to the total amount of poropellent depending on a condition of setting the orbital elements because there occurs a case where the orbit control propellent of approximately 75% must be used at maximum for controlling the argument of perigee to nearly 270 degrees. Thereby, devices mountable on the artificial satellite may be reduced, or on-orbit lifetime of the artificial satellite may be shortened.
An object of the present invention is to set an argument of perigee of one of the six-orbital elements at a setting stage of the orbital elements of an orbit on which the artificial satellite travels in order to solve the above-mentioned problem.
(b) Problems with Regard to Orbit Configuration Methods for a Plurality of Artificial Satellites, and an Object of the Present Invention
Many communication systems which use an elliptical orbit of about a 270-degree argument of perigee and about an 8-hour orbital period are proposed. Apogees of these systems appear above three areas of Europe, North America and Japan, and an object of these systems is to provide communication services using three or six artificial satellites. It can be supposed that the number of the satellites, that is, three or six is intuitively or naturally determined so that three of the artificial satellites come around in the sky above the three areas at a time, respectively. The references do not describe any case where different number of satellites is employed. Further, as to methods of setting the orbital elements, most of the references do not describe any specific numerical values except for a semimajor axis of 20,270 km which is mathematically derived from an orbital period of 8 hours and an orbital inclination angle of 63.4 degrees which is considered to be stable in orbit kinetics. In addition, there is no description on the method of deriving the values.
When an orbit of an artificial satellite is mapped on the ground, what can be expressed geometrically and visually are only four orbital elements of an orbital semimajor axis or orbital period, an eccentricity, an orbital inclination angle and an argument of perigee. Therefore, in a stage of preliminary conceptual design, it is sufficient to set these four elements. This is considered one of the reason why the orbital elements are not clearly described and the deriving method is not described.
In Japanese Patent Application Laid-Open No.11-34996, a method of setting orbital elements of an artificial satellite having a satellite orbital period of approximately 12 hours or 24 hours is proposed. Further, there is description on values of orbital elements of a satellite and number of satellite in a case of service target area of Japan and an elevation angle above 70 degrees. However, there is no proposal on numerical values of orbital elements and number of satellites for orbital periods other than the above-mentioned orbital periods. What is described in the above-mentioned reference is a method of setting orbital elements of an artificial satellite on an elliptical orbit having a long stretch of time staying in the zenith direction in a specified area, and the method can not be applied to all the cases of setting orbital elements of an artificial satellite.
Further, description will be made below on systems using a plurality of artificial satellites which are proposed now or have been developed now. In a mobile communication satellite system, the service target is global, and the satellite travels on a circular orbit (zero eccentricity) having a constant semimajor axis and a constant orbital inclination angle, but the other orbital elements and the method of deriving them are not disclosed. The reason why this is not disclosed may be that they think this belongs to the know-how of the inventors proposed the system using the artificial satellites. An earth survey satellite system developed is a combination of satellites traveling on a sun synchronous semi-tropical orbit so as to survey all over the world. On the other hand, in a case where communication service or surveillance is concentratively and continuously performed to a specified area, stationary satellites are used.
In a case where communication service or broadcast service is intended to be concentratively and continuously performed using an arbitrary number of artificial satellites to a specified area on a celestial body of the satellites traveling around, or in a case where a specified area on a celestial body of the satellites traveling around or a weather condition of the area is concentratively and continuously observed, an object of the present invention is to provide a method capable of being generally applied to setting of orbital elements of the artificial satellite, particularly, a method of setting an orbital semimajor axis, an eccentricity, an orbital inclination angle, an argument of perigee, right ascension of north-bound node and true anomaly of the arbitrary number of artificial satellites, and to provide detailed numerical values of the orbital elements obtained from orbit design according to the method.
(c) Problems with Regard to Mobile Communication and Broadcast to a Mobile Object, and an Object of the Present Invention
It is clear that the existing communication infrastructures such as common line telephones, cellular phones and personal handy phones can not cope with large volume communication to mobile objects.
The stationary satellite communication system likely to cause communication interruption by artificial structures and natural geographical features can not cope with large volume communication to mobile objects.
It is clear that a satellite communication system using a low-to-middle altitude orbit such as Iridium currently under development can not cope with large volume communication to mobile objects because the duration of time while the satellite comes and stays visible in a high elevation angle is as short as several minutes.
The various kinds of communication systems described above can not sufficiently cope with the communication to the mobile objects, but their applicability to digital television broadcast and digital voice broadcast to the mobile objects is negative.
In the broadcast service to the mobile objects using an artificial satellite, the artificial satellite must stably stay visible in a high elevation angle for a long duration of time within a service target area.
The words xe2x80x9can artificial satellite is visiblexe2x80x9d in the present specification is determined that xe2x80x9can artificial satellite stays within a spatial area, under the condition of which communication between an artificial satellite tracking and controlling ground station, various kind of satellite communication send and receive facilities and an artificial satellite can be performed with electromagnetic wave of lightxe2x80x9d.
In order to realize the above, it is generally thought that an elliptical orbit of which the apogee stays in the sky above a service target area is preferable, but appropriate methods and algorithms of setting the orbital elements are not firmly proposed except for in Japanese Patent Application Laid-Open No.11-34996.
In the orbital elements proposed in Japanese Patent Application Laid-Open No.11-34996, the minimum value of eccentricity is 0.24. Even if the value of eccentricity is employed, the distance from the ground to the satellite is generally larger than the distance from that position to a stationary satellite. Therefore, there are the following problems to be solved.
(1) Free spatial loss on electromagnetic wave transmission becomes large, and accordingly the communication/broadcasting devices mounted on the artificial satellite are required to have higher sending and receiving capability. In more detail, in the artificial satellite side, a larger antenna or a sender having a larger output power and a receiver having a higher receiving capability are necessary. In the ground side, a send and receive facility for satellite communication similarly requires a larger antenna or a sending unit having a larger output power and a receiving unit having a higher receiving capability.
(2) Communication delay becomes larger because the distance from the ground larger.
Further, the distance to the artificial satellite in service becomes different between one end part of a service target area and the other end part in the opposite side because the eccentricity is somewhat large. Thereby, when the artificial satellite in service is switched, breakdown time may occur in broadcasting.
In order to solve the above problems, the present invention improves the orbital elements proposed in Japanese Patent Application Laid-Open No.11-34996 from the viewpoint of xe2x80x9ccommunication with groundxe2x80x9d. An object of the present invention is to set more effective ranges of orbital elements within the specified service area of Japanese territory.
Although an object of the present invention is to individually solve the problems described the above items (a), (b) and (c), the object is also to solve combinations of the items (a), (b) and (c), or all of the items (a), (b) and (c) at a time. Another object of the present invention is to provide a method of deriving orbital elements of artificial satellites capable of making mobile communication and mobile broadcast easy to the specified area of Japanese territory using a plurality of artificial satellites by solving the items (a), (b) and (c) together, and at the same time to express the orbital elements suitable for the Japanese territory by limiting the ranges.
Furthermore, a further object of the present invention is to construct various kinds of systems utilizing a plurality of artificial satellites after solving the problems described in the above items (a), (b) and (c).
Description will be made below, corresponding to each of the above items (a), (b) and (c).
(a) With Regard to Setting of One of Orbital Elements of an Argument of Perigee
In a plan using an artificial satellite, a duration to be operable of the artificial satellite is general defined as a mission lifetime. It is necessary to accurately estimate over the period of the mission lifetime using a computer how mach an argument of perigee changes from an initial value of the argument of perigee just after the artificial satellite is injected onto an orbit the satellite should be injected in order to attain its purpose under the condition that the argument of perigee is not controlled at all.
In order to attain the above objects, it is assumed in the present invention that the initial value of the argument of perigee of an orbital element of an orbit on which the artificial satellite travels is set using the above-mentioned estimated value.
In order to attain the above-mentioned objects, the present invention uses an artificial satellite comprising an attitude sensor for detecting its own attitude, a computer for processing detected attitude data, an actuator or gas jet unit for maintaining or changing the attitude using the computer, a gas jet unit for changing its own orbit, and a communication unit for establishing a communication line between the artificial satellite and a control station with electromagnetic wave. In a case where a center celestial body of the artificial satellite traveling around is the earth, the artificial satellite may comprise a unit for receiving electromagnetic wave from a GPS satellite and calculate its own position and velocity. Here, the GPS satellite is a generic name for Navster satellite composing Global Positioning System (GPS) of the USA, Glonass satellite of Russia for navigation, transportation multipurpose satellite of Japan and so on.
(b) With Regard to Orbit Arranging Method for a Plurality of Satellites
In order to attain the above-mentioned objects, in the present invention, an orbit on which an artificial satellite travels is defined by six orbital elements obtained under input conditions of a specified area supplied with service using the artificial satellites, number of the artificial satellites, frequency of service by the artificial satellites to the service target area, a duration time of service by one of the artificial satellites to the service target area and reference time defining the orbital elements.
In more detail, at defining the orbital elements, the six orbital elements are determined by a process of defining number of artificial satellites, a process of defining an orbit semi-major axis, a process of setting an eccentricity, an orbit inclination angle and an argument of perigee, a process of setting a right ascension of north-bound node, a process of setting a true anomaly, and repeating of all the processes from the process of defining number of artificial satellites to the a process of setting a true anomaly.
In order to attain the above-described object, the artificial satellites similar to the above item (a) may be employed in the present invention.
(c) With Regard to Mobile Communication and Broadcast to a Mobile Object
In order to attain the above-described object, the artificial satellites similar to the above item (a) may be employed in the present invention.
Further, in order to attain the above-described object, the present invention employs a group of artificial satellites composed of three or four artificial satellites traveling on three or four elliptical orbits with an orbital period of 24 hours, wherein each of the orbits is formed so that an orbital inclination angle is larger than 37 degrees and smaller than 44 degrees and an eccentricity is not larger than 0.24, or so that an orbital inclination angle is larger than 40 degrees and smaller than 44 degrees and an eccentricity is larger than 0.24 and smaller than 0.35. Therein, each of the artificial satellites is arranged on each of the orbits. Here, with regard to the orbital period of 24 hours, the words, 24 hours, in this specification are defined as a time duration including an error of xc2x110 minutes to 23 hours 56 minutes.
The followings are means commonly used in the above items (a), (b) and (c).
In order to attain the above-described object, in the present invention, the artificial satellites traveling on the orbits in accordance with the present invention are used in systems using various kinds of satellites such as an orbit control system for controlling the orbit of the artificial satellite, a satellite communication system for performing satellite communication through the artificial satellite, an earth survey system using the artificial satellite mounting an earth survey unit and the like.
A satellite communication send and receive apparatus in the satellite communication system is a unit comprising a send and receive means for performing signal sending and receiving with the artificial satellite when the artificial satellite in accordance with the present invention is used within a service target area, and may be mounted on a mobile object moving within the service target area. Further, the send and receive apparatus may be equipped with a GPS means for measuring at least its own position by receiving an electromagnetic wave from a GPS satellite composing a global positioning system, or a measuring means for measuring a consumed amount of substances relating to charges for public services such as electricity, city gas and city water.
Furthermore, in order to attain the above object, in the present invention, the satellite communication system for performing satellite communication through an artificial satellite comprises at least the artificial satellite, a satellite communication send and receive apparatus for performing satellite communication through the artificial satellite and a base station for performing communication with the satellite send and receive apparatus through the artificial satellite. The artificial satellite is an artificial satellite traveling on an elliptical orbit. The satellite communication send and receive apparatus is mountable on a mobile object, and comprises a send and receive means for performing sending and receiving of signals with the artificial satellite when the artificial satellite is used within a specified target service area.
Still further, in order to attain the above object, in the present invention, the satellite communication system for performing satellite communication through an artificial satellite comprises at least the artificial satellite, a plurality of satellite communication send and receive apparatuses for performing satellite communication through the artificial satellite. The artificial satellite is an artificial satellite traveling on an elliptical orbit. Each of the plurality of satellite communication send and receive apparatuses comprises a send and receive means for performing sending and receiving of signals with the other satellite communication send and receive apparatuses through the artificial satellite. At least one of the plurality of satellite communication send and receive apparatuses is located within the above-described target service area though the others are located outside the target service area, and located at a position capable of performing satellite communication through the artificial satellite. Depending on an elevation angle of the artificial satellite when the artificial satellite is seen from the target service area, a relaying mode is selected among a relaying mode between the satellite send and receive apparatuses positioned within the target service area; a relaying mode between the satellite send and receive apparatus positioned within the target service area and the satellite send and receive apparatus positioned within the other area; and a relaying mode between the satellite send and receive apparatuses positioned at positions outside the target service area.